The present invention relates to spread spectrum communications and more particularly to a low power consumption code division multiplex communications system.
While other multiplex communication systems (FDMA, TDMA) cannot permit more than a predetermined number of users, in a code division multiple access (CDMA), since the quality of communication gradually deteriorates (graceful degradation), users can be accepted as long as the code synchronization can be set so that increase in the number of users can be expected. The CDMA has excellent interference resistance, signal concealment, and fading resistance and is being used in a wide range.
According to the CDMA communications system, in a transmitter, baseband data to be transmitted is multiplied by a spread code and further by a carrier, and resultant data is transmitted from an antenna. In a receiver, a spread code having the same phase as that of the spread code at the time of transmission is prepared and the baseband data is decoded by using a correlator.
Hitherto, sliding correlator, SAW (Surface Acoustic Wave) matched filter, digital LSI matched filter, and the like are known as correlators.
According to the sliding correlator, the spread code is cycled faster than a reception signal and a pull-in is performed by a discriminating circuit having a DLL (Delay Locked Loop) or the like. A signal obtained by eliminating carrier components by a sync detector or equivalent means, that is, of a frequency which is about the chip rate is inputted to the sliding correlator. The sliding correlator needs chip synchronization and has drawbacks that it takes time to capture synchronization and that the reception signal including carrier components cannot be inputted to the sliding correlator.
In the SAW matched filter, chip synchronization can be obtained at high speed. Although it can be used in the RF and IF bands, there are drawbacks that since the spread code is decided by a physical pattern of an SAW device, it is difficult to change the code and the filter does not easily correspond to a long spread code.
In the digital LSI matched filter, the chip synchronization is unnecessary. Although there is an advantage that the spread code can be easily changed, there is a drawback of a large power consumption. In the digital LSI matched filter according to conventional CMOS integrated circuit techniques, since the operating speed is slow, there is a drawback that it can be generally used only in the baseband.
In recent years, a mobile communication (portable telephone and the like) is being widely spread. As a communication system employed by the mobile communication, attention has been paid most to the above-mentioned CDMA. The correlator of the CDMA used in the mobile communication is requested to have programmability of the spread code and small power consumption.
However, the SAW matched filter has a problem regarding the programmability of the spread code. On the other hand, the digital LSI matched filter has a drawback of a large power consumption.
Recently, a correlator using a switched capacitor system has been developed and is being put into practical use. The correlator is accomplished by further improving the digital LSI matched filter and has power consumption of about 1/10 of that of the digital LSI matched filter.